The bracket on the Omega is prone to fatigue failure, often leading to one horn falling off, taking the plug with it...
op said: "Just putting all the paperwork in the service file for the Hyundai which had its MOT yesterday"
a more helpful suggestion might be to try a less greedy fussy mot place.
I can read
Some cars have the two horns mounted separately, some on substantial brackets and some, like the Omega, on a single bolt through two thin pieces of steel, which break.
IF the horn is present, then it will be either a wiring issue, a faulty horn or an earthing fault... which depends on how it is wired
A free test, before ordering one, is to swap the horns over and see if the tone changes. If it does then it's an electrical problem. If it doesn't then the horn is toast